Not Applicable.
The present invention is directed to printed wiring boards (PWB""s) having through holes, or other substrates, made electrically conductive by applying a conductive coating containing carbon, and more particularly graphite. The invention is more particularly directed to methods of avoiding blowhole formation in printed wiring boards by treating their through holes with compositions comprising a pH buffer, binders, binding agents, dispersing agents, and combinations thereof, that improve the adhesion and coverage of a conductive coating containing graphite to a surface defined by at least one through hole bore or other substrate. (xe2x80x9cThrough holesxe2x80x9d as used herein refers both to through holes and to vias.)
Conductive graphite and carbon black dispersions are used to provide a superior conductive coating on through hole walls and other nonconductive surfaces. Such dispersions, methods for using such dispersions to coat through holes, and improved printed wiring boards fabricated by using such dispersions are defined in U.S. Pat. Nos. 5,476,580 and 5,389,270, respectively issued to Thorn et al. on Feb. 14, 1995, and Dec. 19, 1995. Both patents referred to in the preceding sentence are incorporated herein by reference in their entireties. A graphite composition, cleaners, conditioners, and other materials and directions needed to practice these patents are available under the trademark SHADOWS(copyright) from Electrochemicals Inc., Maple Plain, Minn. Other carbon dispersions containing carbon black or graphite are described, for example, in U.S. Pat. No. 5,139,642.
Soldering is carried out by coating the through hole walls and other conductive surfaces of a printed wiring board with hot, molten solder to make electrical connections by wetting and filling the spaces between the conductive through hole surfaces and the leads of electrical components which have been inserted through the through holes. A properly soldered through hole is filled with solder.
A problem with xe2x80x9cblowholesxe2x80x9d occasionally develops after the through hole wall has received a conductive coating, has been electroplated with copper, and is then suddenly heated, as by contacting it with molten solder. Soldering heats the copper plated through hole walls very quickly. If there are any gaps or voids in the plated copper, moisture in the substrate is vaporized by the hot solder, which can blow some or all of the solder out of the hole and breach the copper layer. The result is a blowhole or a partially-filled or empty hole, any of which is counted as a soldering defect. It is important that voids do not develop in the copper plate.
The problem of blowholes in through holes made electrically conductive by electroless plating, and the solution to blowholes when that technology is used, are described in a series of articles published in CIRCUIT WORLD, Vol. 12 No. 4 (1986), Vol. 13 No. 1 (1986), and Vol. 13 Nos. 2-3 (1987), under the common title, Blowholing in PTH Solder Fillets. A related article is C.Lea, The Harmfulness of Blowholes in PTH Soldered Assemblies, CIRCUIT WORLD, Vol.16, No.4, (1990). All the articles in this paragraph are incorporated herein by reference in their entirety for their discussion of blowholes in electroless copper technology.
Electroplating copper to the bore of through holes presents several problems as described in U.S. Pat. No. 5,725,807. Chief among these is the requirement for a conductive through hole with a coating of negligible resistance, uniform thickness, durability, and the ability to withstand solder shock tests. This requirement is met, at least in part, by applying an electrically conductive particulate coating of carbon to the through hole. The carbon of the particulate coating is either carbon black or graphite. While the particulate coating described in U.S. Pat. No. 5,725,807 provides distinct advantages and improvements over the prior art, there remains the need to improve the quality of the binding of the particulate coating, and more particularly the binding of the graphite particles to the surface of the through hole.
Accordingly, an object of this invention is to provide improvements in the cleaning and conditioning of through holes and glass fibers.
Another object is to provide a printed wiring board conditioner to improve the adhesion of carbon particles to the conditioned surface.
Yet another object is to provide a substrate conditioner that improves the continuity of coverage of carbon particles on a through hole wall or other substrate.
A further object is to provide a method of soldering through holes to produce fewer blowholes.
At least one of these objects is addressed, in whole or in part, by the present invention.
We have discovered that the binding and continuous coverage of carbon particles to a surface defined by at least one through hole is greatly improved by the addition of an ingredient selected from a pH buffer, a binder, a dispersant, or combinations thereof, to a conditioning formulation applied to a substrate before the carbon particles are applied.